Impeaching credibility is an attack on the credibility of an authority or witness based on the knowledge or truthfulness. While it does not of itself directly challenge the claims of a witness, it can be useful in identifying sources whose statements are either not entered into consideration, or which require independent substantiation before given consideration.
Forms of Impeachment
Particularly, relationship to the parties in a conflict, or financial interest in the outcome.
Prior statements or comments in conflict with current testimony.
A demonstration of "bad" character as regards truthfulness.
Capacity to witness or judge evidence.
Inconsistencies within the present discussion.
Especially discussion or testimony which includes facts not included in a prepared written statement or summary. The key is to show that the subject is aware of the information and is aware that it is important enough to be included.
In US Law
Under the United States Federal Rules of Evidence, any party may attack the credibility of any witness. The "BICCC" mnemonic guides evidence: bias, inconsistent statement, character, competency, and contradiction.